How many employees use cannabis regularly vs. those who don't?  Have you investigated all other possible causes of lethargy or tardiness in these employees?  And are you saying that non-cannabis using employees are never slow and never late?  You are asserting a completely biased and unsubstantiated observation as proof of the effects of cannabis.  Did you ever consider the cause of their problems might be within themselves as people and not the cannabis?  Maybe they just don't like or respect you enough to be on time and work hard.